1. Field of the Invention
The present invention relates generally to air-fuel ratio (AFR) controllers or electronic carburetors for engines that operate at stoichiometric or near-stoichiometric air-fuel ratios, also known as “rich burn” engines, and particularly to providing dynamic feedback to the AFR controller to automatically achieve the optimum set point to improve or maximize the effectiveness of a three-way (NSCR) catalyst as it relates to emissions reductions.
2. Related Art
Rich burn gaseous-fueled engines are thought to be a relatively clean and efficient source for power generation, oil and gas compression, water pumps—and even more increasingly with compressed natural gas (CNG), liquefied natural gas (LNG) or propane-fueled engines for cars and trucks. By adding a Non-Selective Catalytic Reduction (NSCR), also known as a three-way catalyst, along with a more sophisticated air-fuel ratio (AFR) control system, such as the control system for an electronic carburetor of Continental Controls Corporation of San Diego, Calif., described in U.S. Pat. No. 8,005,603, which is hereby incorporated herein by reference, non-desirable emissions of NOx (various oxides of nitrogen), CO, and Non-Methane Hydrocarbons can be reduced to what are considered acceptable levels. As the requirements for these “acceptable” levels of emissions continue to become more and more demanding, the window of control for these control systems becomes smaller and smaller.
In AFR controllers for running engines at stoichiometric air-fuel ratios, or rich burn engines, various methods of changing the ratio of the air and fuel mix are generally based on input from a pre-catalyst lambda sensor or oxygen sensor. This input tells the AFR controller when to add or subtract fuel. There is a narrow emissions control window or range within which the engine meets emission requirements. This window is sometimes referred to as the NSCR control window. This control can be satisfactorily maintained if the engine remains in a perfect environment with no changes in operating conditions. However, in practice, the ability to control an engines air-fuel ratio within this window is affected by drift of the oxygen sensors used to provide feedback to the AFR control, aging of the NSCR catalyst elements, temperature of the catalyst and the exhaust gas from the engine, changes in back pressure on the catalyst, and/or many other variables.
As the target for emissions compliance continues to be reduced, the NSCR control window becomes smaller, and maintaining control becomes more and more difficult. If an AFR controller moves the air-fuel ratio to the rich side of the window, carbon monoxide and ammonia levels increase. If the AFR controller moves the air-fuel ratio to the lean side of the window, NOx goes higher. To establish the correct set point, the standard procedure is to run the engine at a given load, monitor the exhaust for these emissions with an exhaust gas analyzer, and note the point where NOx and CO are both within the window of compliance. At that point, the oxygen sensor, in the exhaust output after the catalyst, is measured for the voltage at the optimum control point. That voltage then becomes the desired set point.
The problem with existing AFR controllers for rich burn gaseous-fuel engines is that the properties of the sensors, catalyst, and even ambient temperatures change, such that the desired set point needs to change. Currently, in order to continue to maintain the window of control for low emission systems, a technician must check the engine with an emissions analyzer and re-establish the proper oxygen sensor set point at frequent intervals.
NOx sensors have recently become commercially available, and they hold great promise for helping with emissions control in the future. They are already being widely used on lean burn gas engines and diesel engines (which also run in a lean mode). The problem has been that, when used for measuring NOx for rich burn engines with an NSCR catalyst, the sensor can read NOx, but has a cross-sensitivity to ammonia or NH3. This sensitivity, along with the very noisy signal generated from NOx sensors on a rich burn engine, has made the use of NOx sensors for emissions control of rich burn engines difficult.